Lymphocyte Markers

A lymphocyte is a type of
white blood cell
involved in the vertebrate immune system. There are two broad
categories of lymphocytes, namely the large granular lymphocytes and
the small lymphocytes. The large granular lymphocytes are more
commonly known as the natural killer (NK) cells. The small
lymphocytes are the T cells and B cells. Lymphocytes play an
important and integral role in the body's defenses.

Natural Killer (NK) cells are a part of
cell-mediated immunity and act during the innate immune response.
They can attack host cells that display a foreign (e.g. viral)
peptide on particular cell surface proteins known as MHC class I
molecules. Once they determine a cell is infected, the NK cells
release cell killing (cytotoxic) granules that will destroy the
infected cell. NK cells do not require prior activation in order to
perform their cytotoxic effect upon target cells. Like NK cells, the
T cells are chiefly responsible for cell-mediated immunity whereas B
cells are primarily responsible for humoral immunity. T cells are
named such because these lymphocytes mature in the thymus; B cells
(named for the bursa of Fabricius in which they mature in bird
species) are thought to mature in the bone marrow in humans. T and B
lymphocytes differ from NK cells in that they are the principal
cells involved in the adaptive immune system. These are cell types
that retain a memory of a previous infection so that they can
respond to the same infectious agent quickly upon reinfection. In
the presence of an antigen, B cells can become much more
metabolically active and differentiate into plasma cells, which
secrete large quantities of antibodies. T cells, after they see an
antigen, will also become highly activated and will secrete specific
proteins, such as cytokines and cytotoxic granules, depending on
their subtype or function.